RED BLUFF, Calif. - As the winter rolls in, one thing that growers in California's Central Valley must watch for is damage to their orchard crops from frost.

Nighttime temperatures have hovered near or below freezing several times in the last week, prompting some growers to take such precautions as irrigation and the use of wind machines.

For Tyler Christensen, who grows almonds, walnuts and plums for prunes just outside of town here, the past week's nighttime lows weren't cold enough to cause concern. But he did see minor leaf burn in his young walnut orchard after a cold event earlier this season, even after preventively irrigating his orchards in November.

"The trees were much more active then," Christensen said. "These things are pretty much shut down now and the temperatures have been more tolerable. The main thing is we want to keep the moisture in the tree up without encouraging it to grow."

In the San Joaquin Valley, Hanford reached a low of 25 degrees in the early hours of Dec. 19 after three straight nights below freezing Dec. 14-16. Many citrus growers have been running water and wind machines to keep their orchard temperatures up, but it hasn't been cold enough for long periods of time to be a danger in most orchards, said Paul Story, director of grower services for California Citrus Mutual.

Conditions have been downright balmy compared to last winter, when the citrus industry spent more than $100 million on wind machines, water and the labor needed to run them amid two months of recurrent nighttime lows in the 20s.

"Right now, cold temperatures are better for us," Story said. "We need the cooler temperatures to toughen this fruit up. Continued warm temperatures tend to accelerate maturity of the fruit."

Growers can actively work to protect their orchards when cold weather occurs, the University of California Cooperative Extension advises. Sprinkler application of water on the orchard floor and lower canopy of the trees can raise the temperature by a few degrees, while microsprinklers provide less protection, according to a narrative by UCCE specialist Rick Snyder.

Some growers have been known to use helicopters to mix warmer air layers above the orchard floor with colder air below during radiation frost events.

Orchardists can also protect their orchards ahead of time with proper management, the UCCE advises. For instance, cover crops or floor vegetation more than 2 inches tall will keep air temperatures a couple of degrees colder than closely mowed orchards.

Christensen said he starts withholding irrigation water after harvest, then he irrigates trees and soil in late autumn before a frost to give them moisture.

"With frost, sometimes by the time you realize the tree's been damaged it's a couple of days later," Christensen said. "This time of year we worry about wood burning. In the spring, we worry about wood burning and crop loss."

To determine frost injury, walnut growers take a sharp knife and cut into the outer bark, UCCE farm advisor Rick Buchner has explained. If it's green and white underneath, it's healthy and alive. If it's black, it's been killed by the freeze.

If trees have been damaged, advisors suggest painting them white, which seems to take some of the stresses away and helps them recover.